What can a 5D Mk3 do that a D800/810 can’t do better?
For me, personally, this.
All taken with a 17 TS-E, a lens I use very heavily, and all with some degree of tilt and or shift applied. Oh and many of my images contain vastly more DR than any current sensor, even the mighty Exmor, can record in a single shot. For much of my paid work another 2-3 stops of DR would make not one iota of difference, I'd still have to bracket the heck out of the scene.
You cannot take any of these images, and have them stand up to as close scrutiny or large prints with a Nikon.
I've not used that lens, so can't comment on that.
Why can't you have Nikon prints stand up to close scrutiny? What are you basing that on?
Given the Nikon has almost twice the megapixels, and 2-3 stops more DR in a single frame, this is the work that a Nikon would excel at.
What is the 5D Mk3doing here that a Nikon can't do exactly?
If you bracket for HDR, you'd actually have to bracket less with a Nikon for shots like these.
How many brackets are you taking, and how far apart?
Say you're combining 3 shots, 1 stop apart. That you could do in one shot on an Exmor.
If you're combining 6 exposures on a Canon, then on an Exmor you'd only need 2 to do the same job.
I'm happy to be told what I'm missing here.
I think the point is that merging frames to HDR is merging frames to HDR. Some of the best HDR shots I've seen were made with up to 15 frames spaced 2 stops apart. They don't really look like HDR...but they have fine nuances of excellent, crisp detail in areas that you simply can't get with 14 stops, or even 16 stops (like the interior of an old WWII plane full of ultra deep shadows, and a bright sunlit sky with puffy clouds outside.)
Merging to HDR is what it is...it doesn't really matter if you use 2, 3, or 15 frames. You blend the data together the same way, with the same tools, and it's all largely effortless these days. Throw in the relatively recently discovered ability to load up 32-bit float TIFF images into ACR and do HDR processing with all the power ACR has to offer, and HDR editing (without getting images packed with processing artifacts) is no more difficult than lifting shadows in a single-frame D810 image.
So, the thing you might be missing...is that HDR isn't difficult these days. If you need more DR, take an extra frame or two (or 14). That works in every camera, regardless of it's sensor capabilities. ACR and Photoshop then make short work of pulling and pushing the exposure into something that fits in the 8 stops average that fits into most people's computer screens.
As I've said before, I want more DR. Mostly because I don't want to be wasting 2 stops or more of what I know Canon sensors are capable of (the majority of their problem isn't the sensor...it's what occurs downstream of the sensor, in higher frequency, noisy components in their DIGIC chips.) But, it isn't like I CAN'T have more DR if I really need it, even though I own Canon cameras. Give a 3-frame HDR merge to 32-bit float TIFF in ACR a try some time...you can get up to 20 stops of DR...it's amazing.